Conventionally, a sheet finishing apparatus that is disposed at a lower side of image forming apparatuses such as a copy machine, a printer, a multi-functional peripheral (MFP) or the like and forms a booklet by finishing such as folding or stapling printed sheets of paper is known.
In the folding, the middle of a bundle of paper is pressed by a nip of a pair of folding rollers and a fold is formed thereon. Then, a roller called a fold reinforcing roller moves along the fold while applying a pressure to the fold, thereby reinforcing the fold.
A sheet stack unit (a stack tray) that stacks a plurality of formed booklets is provided in the sheet finishing apparatus. Regarding the bundles of paper in which the fold is reinforced, the booklets which are consecutively discharged from a discharge port of the sheet finishing apparatus are stacked in order on the sheet stack unit.
Since the volume near the fold swells when the fold reinforcement is not sufficient, the thickness of booklets increases. Therefore, even with booklets having the same number of pages, the quantity of booklets which can be stacked on the sheet stack unit decreases, when compared to the quantity of booklets of which the folds are sufficiently reinforced.
Meanwhile, the swelled volume near the fold can be decreased by increasing the number of times of fold reinforcing. For example, by increasing the number of times of the fold reinforcing, which usually reciprocates once along the fold, to two times or more, the fold is sufficiently reinforced and the volume near the fold becomes thin. Thus it is advantageous when stacking a large quantity of booklets on the sheet stack unit. However, if the number of times of the fold reinforcing is increased, the required time for forming one booklet naturally increases and overall throughput is lowered.
For these reasons, there is a demand for a sheet finishing apparatus and a sheet finishing method which can achieve a balance between the quantity of stacked booklets on the sheet stack unit and the throughput.